With the advance of making higher the density and S/N of magnetic recording media such as magnetic tapes, it has recently begun to use magnetic powder having more smaller particle-sizes.
It is generally said that an S/N ratio of a magnetic recording medium is in proportion to a square root of the particle numbers of magnetic powder contained in a recording material relating to image recording/reproducing operations. When coating the same amount of magnetic powder, it is, therefore, more advantageous for improving an S/N if using magnetic powder having more smaller particle-size. It is also advantageous for obtaining higher electromagnetic conversion efficiency when magnetic powder is finely grained and the specific surface area determined by BET method thereof (hereinafter referred to simply BET value) is increased, because the surface of a magnetic layer may be made so smooth as much and a spacing loss may also be reduced.
The surface of a particle is, however, enlarged at the inverse square of the particle-size. Therefore, the particles can hardly be dispersed drastically as the particle-size is made smaller and the dispersion stability thereof is also deteriorated. The above-mentioned troubles will cause the inconvenience such as the deteriorations in the orientation of a ferromagnetic substance and the surface smoothness of a magnetic layer surface and will result in impossibility of obtaining any excellent squareness and S/N ratios.
As for the binders for magnetic tapes, mainly, a polyester type polyurethane and a vinyl chloride-vinyl acetate copolymer have been used conventionally. However, these binders have been unable to display any satisfactory dispersibility. To cope with this problem, there have used such a method that the dispersibility of magnetic powder in a binder is improved by controlling the particle size distribution of the magnetic powder, or that a surfactant is used for a dispersant. Further, there is a proposal for a method in which a binder is modified by introducing thereinto a hydrophilic group such as a hydroxyl group, a phospho group, a sulfo group, or a carboxy group so as to improve the capability for dispersion thereof.
There may, however, be some instances where a satisfactory dispersibility may not be obtained even by making use of such a binder as mentioned above. It has, therefore, been expected to realize a binder having a high adsorptive power to the surfaces of a magnetic powder.